Treat common symbol in executable as definition

Common symbol in executable is a definition, which overrides definition
from shared objects.  When linker sees a new definition from a shared
object, the new dynamic definition should be overridden by the previous
common symbol in executable.

bfd/

	PR ld/19579
	* elflink.c (_bfd_elf_merge_symbol): Treat common symbol in
	executable as definition if the new definition comes from a
	shared library.

ld/

	PR ld/19579
	* testsuite/ld-elf/pr19579a.c: New file.
	* testsuite/ld-elf/pr19579b.c: Likewise.
	* testsuite/ld-elf/shared.exp: Run PR ld/19579 test.

bfd/ChangeLog

@@ -1,3 +1,10 @@
+2016-03-04  H.J. Lu  <hongjiu.lu@intel.com>
+
+	PR ld/19579
+	* elflink.c (_bfd_elf_merge_symbol): Treat common symbol in
+	executable as definition if the new definition comes from a
+	shared library.
+
 2016-03-02  Alan Modra  <amodra@gmail.com>
 
 	* Makefile.in: Regenerate.

bfd/elflink.c

@@ -1172,9 +1172,12 @@ _bfd_elf_merge_symbol (bfd *abfd,
 
   newdef = !bfd_is_und_section (sec) && !bfd_is_com_section (sec);
 
+  /* The old common symbol in executable is a definition if the new
+     definition comes from a shared library.  */
   olddef = (h->root.type != bfd_link_hash_undefined
 	    && h->root.type != bfd_link_hash_undefweak
-	    && h->root.type != bfd_link_hash_common);
+	    && (h->root.type != bfd_link_hash_common
+		|| (!olddyn && newdyn && bfd_link_executable (info))));
 
   /* NEWFUNC and OLDFUNC indicate whether the new or old symbol,
      respectively, appear to be a function.  */

ld/ChangeLog

@@ -1,3 +1,10 @@
+2016-03-04  H.J. Lu  <hongjiu.lu@intel.com>
+
+	PR ld/19579
+	* testsuite/ld-elf/pr19579a.c: New file.
+	* testsuite/ld-elf/pr19579b.c: Likewise.
+	* testsuite/ld-elf/shared.exp: Run PR ld/19579 test.
+
 2016-03-02  H.J. Lu  <hongjiu.lu@intel.com>
 
 	* testsuite/ld-elf/pr19162.d: Skip hppa-*-*.

ld/testsuite/ld-elf/pr19579a.c

@@ -0,0 +1,15 @@
+#include <stdio.h>
+
+int foo[1];
+int bar[2];
+
+extern int *foo_p (void);
+extern int *bar_p (void);
+
+int
+main ()
+{
+  if (foo[0] == 0 && foo == foo_p () && bar[0] == 0 && bar == bar_p ())
+    printf ("PASS\n");
+  return 0;
+}

ld/testsuite/ld-elf/pr19579b.c

@@ -0,0 +1,14 @@
+int foo[2];
+int bar[2] = { -1, -1 };
+
+int *
+foo_p (void)
+{
+  return foo;
+}
+
+int *
+bar_p (void)
+{
+  return bar;
+}

ld/testsuite/ld-elf/shared.exp

@@ -524,6 +524,21 @@ if { [istarget *-*-linux*]
 	    {} \
 	    "libpr2404b.a" \
 	] \
+	[list \
+	    "Build pr19579a.o" \
+	    "" "-fPIE" \
+	    {pr19579a.c} \
+	    {} \
+	    "libpr19579a.a" \
+	] \
+	[list \
+	    "Build libpr19579.so" \
+	    "-shared" \
+	    "-fPIC" \
+	    {pr19579b.c} \
+	    {} \
+	    "libpr19579.so" \
+	] \
     ]
     run_ld_link_exec_tests [] [list \
 	[list \
@@ -580,6 +595,15 @@ if { [istarget *-*-linux*]
 	    "pass.out" \
 	    "-O2 -fPIC -I../bfd" \
 	] \
+	[list \
+	    "Run pr19579" \
+	    "-pie -z text tmpdir/pr19579a.o tmpdir/libpr19579.so" \
+	    "" \
+	    {dummy.c} \
+	    "pr19579" \
+	    "pass.out" \
+	    "-fPIE" \
+	] \
     ]
 }